1. Field of the Invention
The present invention relates to a brake fluid pressure control apparatus for motor vehicles and, more particularly, to a brake fluid pressure control apparatus capable of keeping the motor vehicle stationary with a driver's foot off a brake pedal.
2. Description of the Prior Art
As a prior-art technique of this type, a brake pressure holding device shown in FIG. 7 has been known. In this drawing, when a specific period of time has passed, with the brake pedal depressed by the driver, in the state that the halt of the vehicle is detected by the main control device not illustrated, a solenoid valve 36 inserted in a main a fluid passage 10 connected between a master cylinder 7 and wheel cylinders 8 is closed to hold the fluid pressure in the wheel cylinders 8, thus holding the vehicle in the stationary state even if the driver takes his foot from the brake pedal.
In the above-described prior-art device, it is necessary to press the brake pedal again to apply the fluid pressure to the wheel cylinders 8 in case the fluid pressure in the wheel cylinders 8 has dropped while the brakes are applied.
The master cylinder 7 has a master piston 34. Around this master piston 34 is fitted a master cup 33 produced of an elastic material such as a synthetic rubber, thereby preventing fluid leakage caused by the fluid pressure from the wheel cylinder 8 side.
Provided in the inside peripheral surface of the master cylinder 7, close to the master cup 33, is a relief port 30 for discharging the fluid pressure out from the chamber of the master cylinder 7.
On ordinary brake operation, the brake pedal is depressed to gradually increase the master cylinder fluid pressure, and accordingly increasing the wheel cylinder fluid pressure. The driver, therefore, can obtain a satisfactory brake feel as he further depresses the brake pedal for a predetermined stroke.
However, when applying the fluid pressure again to the wheel cylinders 8 with the fluid held in the wheel cylinders 8, the master piston 34 starts moving toward pressure application. As the master cup 33 has passed the relief port 30, the fluid pressure from the wheel cylinder 8 is directly applied to the master cup 33. Therefore, brake pedal reaction force suddenly increases and a brake feel differs a great deal from that in ordinary brake operation, resulting in a very heavy brake pedal.